Electrochromic devices are devices capable of changing its transmittance of light in dependence of an applied electric signal. Applications of electrochromic devices include among other things architectural windows, information displays, light filters and modulators, rear-view mirrors, sunroofs and windows in vehicles, eyewear, helmet visors, ski goggles, surfaces with variable thermal emissivity or camouflage.
Different types of electrochromic devices are available. Some devices require a continuous supply of electrical voltage or energy to maintain a certain transmittance level, and are typically referred to as self-erasing devices. Other devices are changed from one transmittance state to another by means of electrical signals but maintain essentially the transmittance if disconnected.
A typical electrochromic device of the latter type comprises five superimposed layers deposited on one substrate or positioned between two substrates in a joined together configuration. The central part of the five-layer electrochromic stack is an ion conductor, also referred to as an electrolyte. The ion conductor is in contact with an electrochromic film, capable of conducting ions. On the other side of the ion conductor is an electron and ion conducting counter electrode film serving as an ion storage layer. The central three-layer structure is positioned between electron conducting layers. Such a device is colored and bleached, respectively, by applying an external voltage pulse between the electron conducting layers on the two sides of the stack, causing the electrons and ions to move between the electrochromic layer and the counter electrode layer. The way this voltage pulse is applied to the electrochromic device is crucial to its performance.
In principle, the simplest way of driving an electrochromic device (ECD) is to apply a coloration or bleaching pulse over a certain, specified, interval of time. A typical pulse used is a rectangular pulse, specified by parameters, such as potential and time. In case of coloration, a coloration potential and a coloration time are defined. The changes in transmittance of the ECD are related to the amount of charge supplied to or extracted from the ECD. The duration of the pulses are therefore of importance. Such an approach is presented in the U.S. Pat. No. 4,412,215, where a control method using fixed times for the coloration of electrochromic devices is disclosed. In order to bleach the electrochromic device, a voltage pulse with opposite polarity is applied and a bleaching potential and a bleaching time are defined. The applied voltage has to be adapted to the used ECD. A too large voltage will destroy the ECD, at least when being applied during a longer time.
In practice, a method of switching based on pre-determined time intervals is not useful in all applications for two main reasons. First, the switching speed of an electrochromic device is strongly dependent on the temperature at which the device is operated. Secondly, the switching speed of an electrochromic device may also change upon its lifetime. An old device may therefore have a different switching speed than a new one. The implications of these aspects are that to achieve the same optical transmittance in the colored and bleached states, the coloration and bleaching pulse, respectively, must be of different duration depending on operation conditions and/or device history. In other words, a voltage pulse of the same duration leads to different degree of coloration or bleaching under different conditions.
A specific example of devices exposed for large varying conditions may be a motorcycle helmet visor used in a cold environment compared to one used on a hot summer day. Another example is an electrochromic facade window exposed to temperature changes throughout the day, season or year. Yet another example is a rear-view mirror or a sun-roof in a car.
Most prior art controlling methods for electrochromic devices do not take the aging of the device into account. A new and fresh ECD has other properties than an ECD that has endured several thousands of cycles. Thus, they cannot be controlled with the same set of parameters for an optimal performance.
There have been earlier attempts to solve these problems. A safe way of achieving the correct coloration and bleaching times is to actually measure the transmittance and interrupt the coloration or bleaching when the required transmittance level is achieved. This is e.g. disclosed in the U.S. Pat. No. 5,822,107, where a method combines time control with measurements of physical characteristics such as voltage, current or light transmittance of the glazing. This, however, requires additional means for optical measurements, which makes the system more complex. There may be cases where the transmittance measurement is not possible, such as non-transparent displays. There may also be cases where an optical sensor would be in the line of sight, disturbing the view in a consumer product or the light beam in an instrument.
There are many prior art disclosures presenting different types of control methods. The U.S. Pat. No. 6,404,532 discloses a system and method for controlling an electrochromic device. The system comprises a light source and an optical detector arranged at opposite sides of an electrochromic window for measuring an attenuation of the light. A pulse-width modulated power signal is used as an input to the electrochromic window.
The U.S. Pat. No. 7,133,181 discloses a control system for an ECD capable of estimating the temperature of the ECD without requiring an external temperature monitoring element and then controlling the ECD based, in part, upon the temperature readings. The controller also provides for methods of determining a bleaching and coloration history of the ECD, determining the transmission state of the ECD and applying a holding voltage to maintain the transmission state of the ECD. Control of ECD coloration and bleaching is performed by using pulsed voltage signals.
A further problem with prior art ECD controlling is the risk of damaging the ECD by high voltages. High voltages have in general negative effects on the lifetime, except for short pulses. This is particularly true for applications where large temperature or ageing differences may be present. The published international patent application WO97/28484 describes a safe driving method, based on applying a pre-set constant current, and specifying the voltage limits that may not be exceeded. If a low current value is specified, the method provides a safe operation mode, however, at expense of slow switching speed.